Touch screen devices are widely used in devices such as ATM's, PDA's, computers, or point-of-sale devices to allow human input of information to an electronic system. A touch screen is a device that, when touched, generates signals identifying a location on a screen where contact was made. Underlying the touch screen is a visual medium such as a cathode ray tube or liquid crystal display tat displays an image. The signals from the touch screen are provided to the electronic system to relate the point of contact to the image on the display. In addition there are signature capture devices which do not include a display beneath the touch screen or touch pad.
An example of an application where a touch screen is commonly used is in an ATM. The ATM is typically placed in a location that is accessible to a wide number of people. People use the ATM to perform banking functions such as depositing, withdrawing, and verifying account balances of a personal bank account. One method for entering the account number to the system is through a card reader on the ATM. The card reader reads the account number off of a magnetic strip on the bankcard. As protection to the user, the account is password protected to prevent a non-authorized person from access. The password is often a combination of digits known only by the user and is entered using the touch screen on the ATM. The user sees a number pad on the ATM display. The user touches the numbers on the pad corresponding to the password. The touch screen sends signals to the ATM that describes the location touched on the display. The ATM converts the locations touched on the touch screen and identifies them to the numbers on the number pad shown on the ATM display. The user will be allowed to access the account if the numbers entered matches the password. Similarly, touch screen devices are also frequently used to take electronic signatures at the point of sale. Everyday examples include writing signatures on a touch screen pad at a retail shop or at an ATM machine after desired transactions are complete.
In general, a touch screen is implemented in four different ways: capacitive, magnetic, surface acoustic wave, and resistive.
In a capacitive touch screen system, a charge storage layer is formed on the touch screen. Touching the screen, for example with a finger, transfers charge to the user thereby decreasing the charge on the charge storage layer of the touch screen. The decrease in the amount of charge (due to the contact) is measured by sensors located at each corner of the screen. A microcomputer receives the signals from the sensors and calculates the coordinates where contact has occurred from the relative differences in charge at each corner and relays that information to the touch screen driver software.
In magnetic based touch screen systems, a grid of magnetic energy is propagated in the X-Y dimension. An example application for a magnetic based touch screen is for capturing a signature. An active stylus is used to write and capture a signature. Information from the active stylus is provided to a microprocessor that reproduces the X and Y coordinates corresponding to the signature for use by the system.
A surface acoustic wave touch screen uses a transmitting transducer and a receiving transducer placed along the x and y axis of the top layer of the touch screen to determine a location of contact. Reflectors are also placed on the top layer to reflect an electrical signal sent from one transducer to another. The receiving transducer can tell if the wave has been disturbed by a touch event at any given instant and can pinpoint its position accordingly.
A resistive touch screen is probably the most widely used and cost effective touch screen on the market today. A resistive touch screen comprises a scratch-resistant layer that protects two conductive layers held apart by spacers. The scratch-resistant layer and the conductive layers are transparent to allow viewing of the underlying display. In a simplified model of a resistive touch screen each conductive layer is modeled as a resistor. An electrical current is conducted by the resistive touch screen when a voltage is applied across a conductive metallic layer. In general, one conductive layer of the resistive touch screen is operated such that current flows in the x-direction while the other conductive layer is operated such that current flows in the y-direction. When an object touches the screen, the two conductive layers make contact creating a bridge resistance between the x and y axis. An approach for determining the point of contact on the resistive touch screen is to separately sense a voltage at the point of contact in the x-direction and the y-direction. Each conductive layer is a resistor divider, sensing the voltage at the point of contact allows the position to be calculated. Knowing the position on the resistive touch screen in both the x and y direction identifies the location of contact.
One problem associated with resistive touch screens is the rapid switching between the conductive layers that is required to read the location at the point of contact. Switching allows either conductive layer to be biased and a voltage sensed. The rate of switching and sensing of either conductive layer is selected to ensure that both the x and y coordinates can be calculated within a normal time period associated with a person touching a touch screen. The switching circuitry adds complexity to the design. Moreover, the switching itself generates noise and voltage spikes that are troublesome to the electronics interfacing with the resistive touch screen and can also result in inaccurate measurements.
A second problem for a resistive touch screen is security when used for a secure transaction such as an ATM or point of sale verification. In particular, there is the threat that the wires coupling the resistive touch screen to a main printed circuit board (system circuitry) could be monitored. The change in voltage on these wires could be detected easily thus allowing someone to steal the information being input. For example, eavesdropping can be achieved by parallel connection of a voltmeter to the wires or by inductively sensing the change in voltage. This is a major security concern for transfer of signatures or PIN's from resistive a touch screen pad to other electrical devices.
Accordingly, it is desirable to provide a resistive touch screen pad that does not require noise generating switching to extrapolate position and pressure at the point of contact. In addition, it is desirable to ensure secure data transfer by making eavesdropping between a touch screen device and other electronics difficult. It would be of further benefit to provide a touch screen-sensing scheme that can operate at a low voltage to achieve energy savings in the device. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.